Scientific Forecaster Discussion
fxus63 kmpx 180020
Area forecast discussion
National Weather Service Twin Cities/Chanhassen Minnesota
620 PM CST Thu Jan 17 2019
Update...for 00z aviation discussion below
Short term...(this evening through Friday night)
issued at 322 PM CST Thu Jan 17 2019
The piece of energy expected to bring snow across southern
Minnesota Friday has reached the western rockies this afternoon.
Snow will begin over Montana and the western Dakotas this evening,
spreading east to southwestern Minnesota by early Friday morning
and across the remainder of southern Minnesota during the mid
morning. This band will be frontogenesis driven and a deep dgz
layer will help sustain decent rates for most of the day south of
the Minnesota river. The question is, and always seems to be, how far
north the snow will get before dry air wins out. Global guidance
and even the ensembles have shifted north today, with all but the
NAM supporting 1 to 2 inches even up to the Metro. This is still
an unlikely scenario given the degree of dry air in the boundary
layer. The gradient will almost certainly be tighter than those
models indicate. Still, cannot completely ignore the recent trend,
so did increase pops some up to I-94, with a dry forecast
remaining north of that corridor. The snow will exit to the
southeast Friday evening.
Total accumulations will depend on a number of factors and is
particularly sensitive due to the unusually wide range of
potential snow ratios. Given how cold the atmosphere is and how
deep the dgz is, ratios of 15-17:1 seem reasonable. However, if
the area of best lift is transient and snow production becomes
less efficient, ratios may be lower than that due to smaller flake
size. Quantitative precipitation forecast across the far south is impressive and ranges from
1/3-1/2 inch. If those ratios do materialize, then it shouldn't be
much of an issue to get 5 to 8 inches southwest of the Minnesota river.
Conversely, if ratios are more 12:1 or so, then we may be talking
closer to 3 to 7 inches. Upgraded parts of the advisory to a
Winter Storm Warning following collaborative efforts with wpc and
the surrounding offices and expected the advisory slightly
northward as well.
Long term...(saturday through thursday)
issued at 322 PM CST Thu Jan 17 2019
Overall, the long term period still looks to be cooler than
normal with widespread light snow looking likely for next week.
By Saturday, we'll be in the wake of the departing system with
northerly flow and temperatures a solid 10 to 20 degrees below
normal. The morning will start out cold, with temps below zero in
western and central Minnesota, and wind chills hovering near the -25
degree mark to start the day across much of western Minnesota.
By late Saturday night, an approaching 850mb trough with decent
Theta-E advection and isentropic lift along with an upper level
vort Max will slide southeast across much of western/southwestern
Minnesota and bring about an inch or so of snow to that area.
Then attention turns to a positively tilted upper trough that
will dig across the western Continental U.S. On Monday. A surface cyclone
will emerge across Colorado and translate eastward through Tuesday. An
inverted trough extending northward into Minnesota will be supported by
upper level vorticity advection and decent differential divergence
thanks to the left exit region of a strengthening jet moving
through the region. This still looks like a light snow event
across a broad area.
Deep northwest flow looks to follow into middle of next week with
below normal temperatures and the opportunity for a quick-moving
clipper system to move through late next week.
Aviation...(for the 00z tafs through 00z Friday evening)
issued at 620 PM CST Thu Jan 17 2019
Snow and IFR/MVFR conditions will move into southern Minnesota
on Friday. Just how far north those conditions get is the primary
forecast challenge this taf period. Krwf and kmkt are close to a
slam dunk for getting snow with IFR/MVFR conditions. At this point
however we anticipate a fairly tight gradient to the snow and reduced
conditions; kmsp could quite possibly miss out on the event
altogether. Northwest winds will swing over to the northeast and
increase above 10 knots on Friday afternoon.
Kmsp...have stuck with the prob30 for snow at this point given the
low prob/confidence. If snow reaches as far north as kmsp the most
likely attendant conditions would be MVFR.
/Outlook for kmsp/
Sat...VFR. Wind north-northwest 10-15 kts.
Sun...MVFR psbl. Wind southeast 5-10 kts.
Mon...MVFR with chc -sn. Wind southeast 10-15 kts.
Minnesota...Winter Storm Warning from 6 am to 10 PM CST Friday for mnz074-
Winter Weather Advisory from 6 am to 10 PM CST Friday for mnz067-
Winter Weather Advisory from 4 am to 6 PM CST Friday for mnz054-